Purification of process gas streams by the removal of acid gases such as CO.sub.2 and H.sub.2 S is required in many major industrial processes such as hydrogen manufacture for refinery hydrotreating, synthesis gas manufacture from coal, purification of natural gas, and ammonia manufacture. Bulk removal of acid gases from such streams is usually accomplished by use of a scrubbing solution composed of a chemical or a physical solvent. The characteristic differences between chemical and physical solvents are that the chemical solvents chemically react with the acid gases during the absorption process. Chemical solvents generally have a relatively high heat of absorption of acid gases and they usually require heating for their regeneration. On the other hand, the physical solvents absorb acid gases by dissolution without chemical reaction. Physical solvents generally have a relatively low heat of absorption of CO.sub.2 and usually require only depressurization for their regeneration. The capacity of physical solvents to absorb gas is approximately proportional to the partial pressure of the gas being absorbed. As the acid gas pressure increases from about 80 to 300 psig or more, the capacity of the physical solvents to absorb gases surpasses that of chemical solvents. In general, the higher the partial pressure of acid gas the greater the capacity advantage of physical over chemical solvents.
Physical solvents are increasingly being considered for use because many new gas manufacturing processes utilize higher pressures. Furthermore, because of the possibility of increasing energy costs, physical solvents have an economical advantage because they can be regenerated by depressurization as opposed to heating. However, because of the nature of true physical solubility, which consist only of weak interaction between gases and solvent, physical solvents have limitations such as: (1) high affinity for absorbing hydrocarbons, (2) low acid gas solubility at mode-rate acid gas pressure, and (3) low H.sub.2 S/CO.sub.2 selectivity.
Consequently, there is a need in the art for solvents capable of absorbing acid gases which are not limited by the limitations of the conventional physical solvents but which can still be regenerated by depressurization.